Presented to Star Astronomy May 1, 2008 By Charles J. Byrne –Image Again – With topographic views from Nick Lordi The Near Side Megabasin of the Moon

The Early Moon A Mars-sized body, its orbit perturbed, collided with Earth, 4.5 Billion years ago. This body, and part of Earth’s crust, were vaporized and formed a ring around Earth. Soon, this ring cooled, and the Moon accumulated. The heat released by gravity formed a magma ocean.

Crystallization of the Early Moon

Asymmetry of the Current Moon The “Man in the Moon” (maria) is mostly on the near side The near side is low, a bulge on the far side The crust is thin on the near side, thick on the far side Heavy element anomalies are mostly on the near side Uneven moments of inertia: offest C. G.

Near Side of the Moon

Far Side (Nozomi)

Eastern Limb (Lunar Orbiter)

Eastern Limb (Apollo 16)

Maps of the Current Moon Topography –Photography (photometry and stereo) –Laser and radar altimeters Gravity potential –Tracking of spacecraft Crustal thickness: inferred from topography and gravity Mineral concentrations

Clementine Elevation Map m 5000 m 0 m

Why? Asymmetric impacts? Uneven crystalization of the magma ocean? Tidal effects in a complex early orbit?

Impact Dynamic Tutorial Incoming asteroids and comets Hypervelocity impacts: explosions Cavity formation Ejected target material Formation of rings Effects of target curvature Scaling laws

Lunar Basin (Orientale)

Orientale, LIDAR and ULCN (Lordi)

Noise Reduction for ULCN (Lordi)

Orientale, ULCN (Lordi)

Orientale, radial profile

Simulation of Impacts

Radial Profile of Selected Basins

Profile of Ejection Velocity

Curvature of the Target A giant basin must consider the spherical nature of its target Ejecta is thrown into elliptical orbit There is less area for the ejecta to land near the antipode, so it piles up

Steps in Making the Model “Flat Moon” basin model Ejection velocity radial profile Orbital trajectory equations Focusing effect of the spherical Moon Final radial profile of ejecta

The Search The scaled model has these parameters: –Latitude and Longitude of center –Diameter –Depth Parameters were varied to make a best fit Started with two large basins They grew as the fit improved They merged into one giant basin

Model of the Near Side Megabasin

600 km impactor Escaping ejecta (hyperbola) Escapevelocity

Velocity less than escape Ejectapassesantipode Transient crater expands

Velocity falls further Ejecta is concentrated at antipode Antipode

Velocity falls further Ejecta falls between basin rim and antipode

Radial Profile of the Near Side Megabasin

The NSM Floor is Refilled with Crust

Refilling of Plastic Crust

Model of the Moon with the NSM

The NSM and its Antipode

NSM and Titanium

NSM and Iron

NSM and Thorium

NSM and Maria

NSM Rim on the Far Side

NSM Rim at Tsiolkovskiy

Summary The history of the Moon from its origin to today has been reviewed Its original crust has been strongly modified by impacts, starting with the NSM and SPA. 4 billion years of bombardment followed The major mineral anomalies on the surface are associated with the impacts of the NSM and SPA.